1) Field of the Invention
The present invention relates to an image forming apparatus like a copying machine, a printer, a plotter, or a facsimile and a process cartridge used in the image forming apparatus.
2) Description of the Related Art
In recent years, image forming apparatuses like a copying machine and a printer have been widespread in the market. A color image forming apparatus is also being widespread in the market in accordance with colorization of documents.
In an electrophotographic system that is widespread as a system for an image forming system, a process described below is executed as a representative image forming process. First, a photosensitive member serving as an image carrier is uniformly charged by a charger and, then, exposure corresponding to image information is applied to the charged photosensitive member to cause a potential difference between a non-image portion and an image portion. Then, toner particles are deposited only on the image portion by a developing unit to form a toner image, which is transferred onto a recording medium such as recording paper or an OHP sheet directly or via an intermediate transfer member. When a color image is formed, toner images of respective colors are superimposed one on top of another by various publicly known methods. For example, the image forming process described above is carried out for each color to sequentially form color images of respective colors on a photosensitive member, and the toner images are sequentially transferred on to a recording medium directly or via an intermediate transfer member. Alternatively, toner images of plural colors are formed one on top of another on a photosensitive member to transfer the toner images collectively on to a recording medium directly or via an intermediate transfer member. Alternatively, toner images of respective colors are formed on plural photosensitive members, respectively, and the toner images are superimposed on a recording medium directly or via an intermediate transfer member at the time of transfer. A single color toner image or a color toner image formed on the recording medium is fixed on the recording medium in a fixing unit.
Incidentally, compared with the single color image, the color image is often colored in a background portion as well, which tends to increase a quantity of toner to be consumed for forming one image. The increase in a quantity of toner consumption is unfavorable from the viewpoint of a reduction in an environmental load.
From the viewpoint of an image quality, when a large quantity of toner is deposited on one pixel, a toner layer thickness per one pixel increases. Thus, dust of the toner tends to scatter when a toner image is transferred, and a dot area of the toner image tends to increase when the toner image is fixed. These phenomena occur even in the single color image and occur particularly conspicuously in the color image. As a result, sharpness of the images is hindered, which leads to deterioration of image qualities.
Moreover, in the color image, the number of colors of toners deposited on one pixel is different for each pixel. Thus, a thickness of a toner layer changes for each pixel and a rate of increase in the dot area also changes when the toner image is fixed. When a dot area per one pixel varies, granularity of an image worsens, that is, the image is roughened, which leads to deterioration of the image quality.
As conventional technologies for improvement of granularity according to a control of exposure energy, various systems are disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-118036 and Japanese Patent Application Laid-Open No. 2003-54026. In the system disclosed in Japanese Patent Application Laid-Open No. 2000-118036, output energy of a light beam of an exposure device is controlled taking notice of optical potential attenuation characteristics of a photosensitive member. In the system disclosed in Japanese Patent Application Laid-Open No. 2003-54026, an exposure pattern is selected and used in a highlight portion.
On the other hand, when a developing unit is used for a long period of time, a toner inside the developing unit deteriorates due to mechanical and thermal stresses. In particular, an extraneous additive like silica coating the toner is buried in the toner surface or separated from the toner due to the stresses. This causes a problem in that charging characteristics and flow characteristics of the toner change and an image quality deteriorates. To cope with this problem, a system for preventing aged deterioration of a toner by specifying a shape and a particle diameter of an extraneous additive is proposed as disclosed in Japanese Patent Application Laid-Open No. 2002-196526 and Japanese Patent Application Laid-Open No. 2003-057864.
As effective means for improving an image quality, it is possible to reduce a quantity of toner to be deposited per a unit area of an image portion. In the following description, a weight of toner to be deposited per a unit area of an image portion is called M/A, which is used as a characteristic representing a quantity of toner to be deposited per the unit area.
The reduction in a quantity of deposited toner leads to a reduction in a quantity of toner consumption and a reduction in an environmental load. In addition, the-transfer dust and the increase in a dot area at the time when a toner image is fixed are controlled through the reduction in the quantity of deposited toner, and a dot area difference among pixels is also reduced. Moreover, deficiencies like deformation and curl of a recording medium due to a thickness of a toner layer are also reduced significantly. From such viewpoints, the applicant has been studied an improvement of an image quality and the like at the time when a quantity of deposited toner is reduced.
However, while the applicant carried forward the examination, the applicant noticed that, in an image forming process with a reduced quantity of deposited toner, granularity of an image deteriorated noticeably as an image forming apparatus was used longer, and an initial image quality could not be maintained. In particular, the applicant found that, as an image quality that changed with time, granularity in a highlight portion worsened compared with the initial image quality of the image forming apparatus.
The applicant observed a toner in a developing unit when the toner is in an initial period and when the toner is aged using an electron microscope (SEM). Then, although a state in which an extraneous additive coated the toner surface was observed in the initial toner, no extraneous additive was observed on the toner surface in the aged toner. This indicates that the extraneous additive was buried in or separated from the toner surface in the aged toner due to mechanical and thermal stresses as explained above concerning the conventional technologies.
The applicant carried out an experiment described below to investigate how the aged toner, in which the extraneous additive was buried or from which the extraneous additive was separated, affected an image quality.
First, the applicant prepared two types of developing units in an initial state and an aged state and set the developing units in an image forming apparatus to output images. In the developing unit in the aged state, in which a developer is inside the developing unit, images are created in an accelerated manner by idling of the developing unit with a single driving device for 120 minutes. In this case, the applicant sampled the toner and observed a coating state of the extraneous additive using the electronic microscope. Then, the applicant confirmed that a state of the toner surface was the same as that of the aged toner in the state in which the extraneous additive was buried in the toner surface or separated from the toner surface described above.
As image forming conditions, a resolution was set to 1200 dots/inch (dpi), a charging potential was set to −630 volts, a developing bias was set to −500 volts, a toner diameter was set to 5.5 micrometers, and a carrier diameter was set to 35 micrometers. Conditions for the experiment were set such that a quantity of deposited toner per a unit area M/A in a solid image on paper (a state in which a toner was deposited over the entire surface of the paper) was 0.45 mg/cm2.
Here, assuming that granularity, which was roughness of an image, was caused by fluctuation in a dot area in a half-tone dot, the applicant evaluated the fluctuation in the dot area to use the fluctuation as substitute for the granularity. In addition, to check contribution of deterioration in an image quality in respective processes, as evaluation of images, the applicant evaluated a dot image on a photosensitive member after development, a dot image on an intermediate transfer member after transfer, and a dot image on paper after fixing, respectively. In the evaluation of a dot area, the applicant photographed dot images in the respective processes using a digital microscope and binarized the images to thereby obtain respective dot areas in the half-tone dot. The applicant evaluated a standard deviation of the dot areas as an amount of fluctuation in the dot areas.
FIG. 6 shows an evaluation result in this case. The horizontal axis indicates the respective processes, and “after development”, “after transfer”, and “after fixing” represent an image on a photosensitive member, an image on an intermediate transfer member, and an image on paper after fixing. In addition, the vertical axis indicates a standard deviation σ representing fluctuation in a dot area. From FIG. 6, it is seen that a difference between an initial toner and an aged toner increases after transfer, which indicates that deterioration in an image quality is large in a transfer process when the aged toner is used. Consequently, it is considered that this deterioration in an image quality is promoted even after fixing to worsen granularity.
The applicant assumes a mechanism as described below concerning the deterioration in an image quality in the transfer process of a toner (aged toner) in which the extraneous additive is buried or from which the extraneous additive is separated from. Since the aged toner is coated with the extraneous additive in a small area on the toner surface compared with the initial toner, it is estimated that a non-electrostatic adhesive force of the toner adhering with the photosensitive member is large. Thus, although transfer efficiency falls, usually it is possible to adjust the transfer efficiency according to conditions like a transfer bias. However, in this case, the transfer efficiency is mainly adjusted using a pattern with a large quantity of deposited toner like a solid image. In this experiment, for the initial toner and the aged toner, transfer conditions are already adjusted such that a quantity of deposited toner in a solid portion on paper is fixed. However, since a dot image in a highlight portion has a transfer characteristic different from that in the solid portion, it is considered that proper transfer efficiency is not obtained.
The applicant considers the difference in transfer efficiency according to an image pattern as follows. It is found in conventional measurement or the like that a toner layer consisting of about two to three layers is formed in the toner image on the photosensitive member after development. Since a transfer electric field is applied in the transfer process, a force moving from the photosensitive member in a direction of the intermediate transfer member acts on the toner on the photosensitive member. In this case, the toner image is required to be at least electrostatically transferred with a force stronger than an adhesive force between the photosensitive member and the toner such that the toner is transferred entirely. Here, a simple model as described below is devised. First, in the case of the solid image, in the toner image on the photosensitive member, it is assumed that a toner layer in contact with a photosensitive member (OPC) is A, and a toner layer on the toner layer A is B as shown in FIG. 7A. When the toner layer shown in FIG. 7A is considered, an adhesive force between toners acts on the toner layer A and the toner layer B, and an adhesive force between a toner and a photosensitive member acts on the toner layer A and the photosensitive member (OPC). Usually, a non-electrostatic component is large in the latter adhesive force. Therefore, when the transfer electric field is weak or when a non-electrostatic force is large, the toner layer A portion remains on the photosensitive member in a large quantity. In other words, since the non-electrostatic force is large in the aged toner, a quantity of transfer residual toner in the toner layer A portion is large. Therefore, adjustment is performed such that a target quantity of solid deposited toner by intensifying the transfer electric filed or increasing an input quantity of deposited toner (toner layer B portion). However, it is well known that, to the contrary, application of an excessive transfer electric field deteriorates the transfer efficiency and causes deficiencies like scattering of a toner. Thus, it is necessary to set a quantity of solid deposited toner taking into account a target quantity of deposited toner and a target transfer rate in advance.
On the other hand, in the case of the dot image in the highlight portion, an area of the dot image is reduced and, unlike the solid image, an edge portion of the dot image affects the toner image on the photosensitive drum. Thus, it is considered that the toner layer B has an angle shape as shown in FIG. 7B in the toner image on the photosensitive drum. Although the same action as that in the solid portion acts in the transfer process, when the toner deteriorates and a non-electrostatic adhesive force increases, even if the toner layer is increased at the same rate as that in the solid image, an amount of the increase is small because the toner layer B has the angle shape. Thus, in the aged toner, even if there is a sufficient amount of solid image on paper, a quantity of transferred toner is not sufficient in the highlight dot image. It is considered that this worsens granularity with time.